The present invention relates generally to sensing output current of any rectifier circuit with a capacitor output filter and, more particularly, to improvements in circuitry for sensing the current in a motor control drive circuit for effecting current limiting and obtaining improved short circuit protection.
A conventional, basic motor control circuit 10 illustrated in prior art FIG. 1 receives ac line input power 12, 14 and 16 and converts them to output drive power 46, 48 and 50 for the motor 52. In well known manner, the diodes 18, 20, 22, 24, 26 and 28 rectify the ac line voltage to produce a dc rail voltage which exists between the positive dc bus line 30 and the negative dc bus line 32. This rectified dc voltage is filtered and smoothed by a capacitor 34.
The power switching transistors 35, 36, 38, 40, 42 and 44 comprise a pulse with modulated (PWM) inverter which responds to a control circuit (not shown) which drives the gates of these transistors to produce the aforementioned motor drive signals 46, 48 and 50.
The inverter dc line current generally requires sensing to provide the following four functions:
1) Regulation of normal operating current; PA1 2) Current limiting; PA1 3) Line-to-line short circuit protection at the outputs; and PA1 4) Output line-to-ground fault protection.
With the advent of smaller, more cost-effective power-train layouts incorporating surface-mount IGBTs, the need arises for more compact and more cost-effective ways of deriving the required current feedback signals.
In one known solution, a current-viewing resistor 60 is connected in series with the negative dc bus line 32 of the inverter, as shown in FIG. 2. The current signal at node 62 serves functions 1 through 3 above. The value of the current viewing resistor 60 typically is set so that it develops 500 mV-1000 mV at maximum operating current. The resulting power loss, for maximum operating currents of 10 to 15 amps, is tolerable. At higher currents, however, the power loss in the current-viewing resistor 60 becomes impractical.
The operating current could theoretically be increased by choosing the resistance of the current-viewing resistor 60 so that it develops 100 mV to 200 mV at full operating current, thus keeping the power loss to a tolerable level. A severe practical problem, though, is that the L/R (inductance to resistance) ratio of the current-viewing resistor increases as R decreases. This produces unacceptable distortion of the current signal, because of the high di/dt (rate of current change) of the inverter current. The problem is compounded as operating current increases, because both di/dt and the L/R ratio simultaneously increase.
FIG. 3 shows a comparison of the current signal waveform 64 from a commercially available "low inductance" 5 m.OMEGA. current-viewing resistor, versus the "real" waveform 66 of current. The distortion produced by the current-viewing resistor renders the signal unusable.
In another known solution, function 4 above can be satisfied with a current-viewing resistor 68, a limiting resistor 69 and an opto-coupler 70 in the positive dc bus 30, as shown in FIG. 4. The current-viewing resistor 68 must develop at least 1.0 V to activate the optocoupler 70. This is acceptable for operating currents of up to about 15 amps. At higher currents, the losses in the current-viewing resistor 68 become prohibitive.
Another common solution is to use a current transducer that employs a Hall-effect current sensor 72 in the dc bus 30, as shown in FIG. 5. The Hall-effect current transducer 72 is able to transmit dc, and delivers an isolated current signal 74 which is an acceptable replica of the actual inverter current.
The disadvantages of a Hall-effect current transducer are its relatively high cost (about $10-$15) and size. A typical 50A Hall-effect transducer which would be suitable for a 10hp drive is an LA 50-5 Hall-effect transducer available from LEM S.A. of Switzerland. The overall outline of the above transducer is approximately 1.6".times.0.7".times.1" high, for a total volume of 1.1 cubic inches.
In accordance with still another solution, a Hall-effect transducer can be modified to provide function 4 by using separate primary windings 78, 80 in the upper and lower dc buses respectively, as shown in FIG. 6. The two primaries 78, 80 buck each other, and have different number of turns. For example, if the upper winding 78 has three turns, and the lower winding 80 two turns, then there is one total effective primary turn for normal operation, two for a lower ground fault, and three for an upper ground fault. Logic circuitry (not shown) detects when an upper or lower ground fault occurs.
Again, the disadvantages of this solution are size and cost.